The present invention is broadly directed to an improved apparatus and method for rapidly freezing tissue specimens at cryogenic temperatures that enhances heat transfer, quickly cools tissue holders and tissue permits use of shaped cryogenic surfaces and facilitates cutting of thin tissue sections. More particularly, it is directed to a tissue freezing apparatus having improved cryogenic structure including a channel system permitting highly effective circulation of a cryogen and transfer of heat thereto so as to rapidly cool tissue specimens.
Biopsy or surgical removal of tissue specimens for histologic examination, is commonly employed for diagnostic purposes. When a lesion is known or suspected to be malignant, the entire mass is generally excised, if possible. An examination technique is often preferably employed in which the entire tumor margin surface area is reviewed under a microscope. This technique involves microscopic screening of the exterior surface area of the tumor for the presence of malignant cells in order to ensure that all such cells have been removed. If practiced effectively, tumor margin surface area examination enhances the likelihood of complete removal of all cells of a localized malignancy.
Once harvested, the tissue sample is preferably quickly frozen at a controlled rate using a cryogenic coolant in order to obtain high quality frozen sections suitable for use in diagnosis. The tissue is then cut into thin layers or sections for histological examination. It is important that the tissue be frozen and the histologic examination performed as quickly as possible, since the patient must be kept waiting pending the microscopic evaluation, in case any additional tissue must be excised. In the past each review of the tissue was comparatively very lengthy, so that a patient had to be maintained in a very uncomfortable state with an open wound for a long period of time. Much of the delay was due to slow freezing of the tissue samples, so fast freezing is very desirable, especially where multiple samples must be taken.
Controlled freezing of the tissue may be accomplished using the methods and devices set forth in Applicant""s previous patents, such as U.S. Pat. Nos. 4,695,339; 4,752,347; 5,628,197; 5,829,256; 6,094,923 and 6,289,682, which are incorporated herein by reference. The rate at which specimens can be frozen under such controlled conditions is determined by the rate of heat transfer from a cryogenic fluid, such as liquid nitrogen, to the platform on which the tissue is placed. Specimens must be frozen relatively quickly in order to avoid formation of large water crystals. However, attempts to increase the rate of freezing by use of excessive amounts of cryogenic material may impair control over the freezing process. Specimens that are frozen unevenly or incorrectly may be marred by voids and artifacts that might impair histologic examination and diagnosis. It is also desirable to minimize the quantity of cryogenic fluid that is used, since such fluids are costly and may present certain environmental hazards which must be addressed. Therefore, it is important to enhance heat transfer while maintaining control over specimen freezing conditions and conserving use of cryogenic fluids.
Even a properly prepared tissue specimen that is quickly frozen under controlled conditions may not result in a high quality histologic specimen unless thin tissue sections can be taken easily from the frozen specimen. Compression of the section may occur where difficulty is encountered in cutting thin sections from a frozen specimen. Upon gross examination, compressed tissue sections may appear to be usable for mounting on slides, but will prove to be difficult to evaluate. Badly crumpled sections may be unusable. Compressed and crumpled sections are often produced by the shape of the structure upon which the sections are prepared and frozen.
Accordingly, there is a need for an apparatus and method for evenly and quickly freezing a tissue specimen under controlled conditions while facilitating tissue cutting, conserving cryogenic fluids and providing flexibility in the shape of the cryogenic surface.
The present invention is directed to improvements that enhance heat transfer in an apparatus and method for preparing frozen tissue specimens. The apparatus includes a base supporting a pair of rotary motion platforms and a center platform. The rotary motion platforms are each movable from an open, side-by-side position to a closed, center platform-covering position. Each platform includes a series of cryogenic discs equipped with a channel system for circulation of a cryogenic fluid within the disc structure. The channel system includes a series of radial channels communicating with a peripheral channel. The radial channels each include a series of interior circumferential fins for causing turbulent flow of the cryogen and further increasing heat transfer. The discs have a circumferential ring seal. The discs for the rotary motion platforms each include a central inlet port and a pair of peripheral outlet ports communicating with the channel system for addition and exhaust of circulated cryogenic fluid. The discs of the center platform include peripheral inlet and outlet ports and a central aperture for receiving a tissue holder. In certain embodiments, the discs include a radially extending peripheral flange, which serves to reduce disc thickness and the mass of the material to be cooled. Such a flange may extend selectively to alter the circular shape of the disc. In some embodiments the upper surfaces of the rotating platform discs have a convex configuration for promoting better tissue samples.
Objects and advantages of this invention will become apparent from the following description wherein are set forth, by way of illustration and example, certain embodiments of this invention.